Plant functional traits are useful predictors of their responses to environmental conditions, as well as how they influence their ecosystem. Intraspecific variation in functional traits among sites is driven by non-inherited phenotypic plasticity and evolutionary adaptation. In the case of recently introduced exotic species, local adaptation might be expected to play a relatively minor role. At the same time, understanding the mechanisms of trait variation among exotic species may be useful in predicting their impacts and responses to control measures. We tested these ideas using three widespread exotic grasses in California, by means of in situ field measurements of functional traits and a common garden experiment.
During April-June of 2017, we measured leaf area, height, specific leaf area and seed mass from three grass species (Avena barbata, Brachypodium distachyon, and Briza maxima) across four field sites in Marin County, California (Mount Burdell, Old Saint Hilarys, Terra Linda/Sleepy Hollow Divide and Verissimo Hills). In addition, we collected seed from these sites for use in a common garden study. Twenty seeds of each species from each site (80 seeds total for each grass species) was grown in a common garden experiment to observe variation in growth rate, specific leaf areas and other measurements according to the population of origin. Seeds were sown in October 2017, growth was measured approximately weekly, and leaves were harvested to measure specific leaf area (SLA) in April 2018.
Results/Conclusions
The growth rate of these three exotic grass species (Avena barbata, Brachypodium distachyon, and Briza maxima) differed among populations, particularly late in the experiment. For example, for Avena barbata and Briza maxima there was no significant differences in growth among populations from germination until day 60. Avena barbata populations that were taller in the field also grew more rapidly in the common garden, suggesting a genetic component to the field trait differences. In contrast, Brachypodium distachyon, and Briza maxima showed only weak correlations between field height and common garden growth rate, suggesting that height in these species may be driven more by plastic growth responding to environmental conditions. Germination, survival and SLA also differed markedly among populations. Together, this suggests that local adaptation can occur in exotic plant species, despite a relatively short history of occurrence in the region.